Kotlin: update Gradle wrapper and language version

Kotlin/build.gradle

@@ -1,5 +1,5 @@
 buildscript {
-    ext.kotlin_version = '1.2.21'
+    ext.kotlin_version = '1.3.41'
 
     repositories {
         mavenCentral()
@@ -11,7 +11,7 @@ buildscript {
 }
 
 plugins {
-    id "org.jetbrains.kotlin.jvm" version "1.2.21"
+    id "org.jetbrains.kotlin.jvm" version "1.3.41"
 }
 
 apply plugin: 'kotlin'

Kotlin/gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,6 @@
+#Wed Nov 20 10:18:48 CET 2019
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.8.1-all.zip
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.4-bin.zip
+zipStoreBase=GRADLE_USER_HOME